1. Field of the Invention
The present invention relates to a scanning optical system in which a beam is deflected using a scanning device such as a galvanomirror or a rotary polygon mirror and which permits an increase in the angle of deflection by the single deflecting device itself so as to allow high speed scanning.
2. Description of the Prior Art
It is known in the art of scanning optical systems to arrange a plural number of scanning devices in series so as to obtain a wide scanning angle. However, this known method is troublesome and complicated to carry out since there are used a plural number of individual scanning devices. It is a problem to effect synchronism of rotation between these scanning devices. Further, positional variations caused by inclination (fall-down) of the rotation axes should be taken into consideration.
To eliminate these problems, it has been proposed, for example by U.S. Pat. No. 3,771,850, that the scanning angle be widened by using a single scanning device which deflects a beam plural times on it. According to the method, a beam before traversing a transmission optical system used as a means for widening the angle of deflection and a beam after traversing the transmission optical system are reflected upon the same reflecting surface of the scanning device. This arrangement also has some disadvantages and problems. For example, there occurs an important problem of overlap of the optical paths of the beam before deflection and of the beam after deflection and having a widened scanning angle. This overlapping of the optical paths can not be eliminated without giving up the widening of the scanning angle.
If one makes a beam incident upon the scanning device with an angle between the incident beam and the scanning surface of the device and separates the beam before deflection from the beam after deflection to eliminate the overlap of the optical paths, then the scanning line of the beam after deflection will have a conical shape. Herein, the term "scanning surface of a scanning device" is to be understood as a plane that is formed by the normal of the reflecting surface of the scanning device as the latter rotates.